1. Field of the Invention
The present invention relates to a power amplifier, more particularly, to a power amplifier suitable as a guitar amplifier.
2. Description of the Related Art
In recent years, power amplifiers, such as class D amplifiers, utilizing semiconductor devices have been widespread as audio power amplifiers. For example, in a class D amplifier, the output-stage transistor thereof is turned ON/OFF by a PWM pulse train having been pulse-width modulated on the basis of an input audio signal AIN, and the current from a power source is supplied to a speaker via this output-stage transistor. Since the output impedance of this class D amplifier is very low and consequently the loss of the output-stage transistor is very small, the amplifier can efficiently supply electric power to the speaker. More specifically, the output impedance of the class D amplifier is nearly 0 Ω while the internal impedance of the speaker is approximately 8 to 16 Ω. Furthermore, usually in the class D amplifier, the voltage applied to the speaker is negatively fed back to the input section, and the gain of the class D amplifier is controlled depending on the difference between the input audio signal and the negative feedback signal thereof. Hence, the class D amplifier can perform constant voltage drive in which the voltage corresponding to the input audio signal is supplied to the speaker. For example, WO2003/090343 is available as a prior art document relating to the class D amplifier.
Even in the present day in which such power amplifiers utilizing semiconductor devices as those described above have been widespread, many vacuum tube amplifiers are still used as guitar amplifiers for amplifying the output signals of electric guitars. This is because such vacuum tube amplifiers can emit sound with quality preferred by performers at a large volume from a speaker in comparison with the power amplifiers utilizing semiconductor devices. The following points are taken into consideration as the reasons for this.
FIG. 10 is a graph illustrating the frequency characteristics of the impedance of a full-range speaker. This speaker generally has a resonance frequency (f0) at a frequency of approximately 80 to 100 Hz, and the impedance of the speaker becomes high at this resonance frequency (f0). Furthermore, since the frequency band near the resonance frequency (f0) corresponds to the pitches generated from the fifth and sixth strings of an electric guitar, the frequency band is an important frequency band affecting the sound quality of the electric guitar.
In the case of a power amplifier in which the output voltage is negatively fed back, such as a class D amplifier, that is, in the case of a power amplifier for driving a speaker at a constant voltage, the speaker is driven at a voltage corresponding to an input audio signal without being affected by the frequency characteristics of the impedance of the speaker. Hence, as shown in FIG. 11A, since the speaker is driven at a constant voltage even in the frequency band near the resonance frequency (f0) in which the impedance of the speaker is high, the vibration amplitude of the speaker is constant and the amplitude of the sound to be emitted from the speaker is also maintained constant.
On the other hand, in the case of a vacuum tube amplifier, a pentode is generally used as an amplifying device for output, and this pentode has a characteristic of high output impedance. Furthermore, in a vacuum tube amplifier using the pentode with the high output impedance, the speaker is driven by a constant current corresponding to the input signal supplied to the grid of the vacuum tube, regardless of fluctuations in the impedance of the speaker. Hence, in the frequency band near the resonance frequency (f0) at which the impedance of the speaker becomes high, the voltage of the vacuum tube amplifier for driving the speaker increases as shown in FIG. 11B, whereby the vibration amplitude of the speaker increases and the amplitude of the sound to be emitted from the speaker also increases. As a result, the sound generated from the fifth and sixth strings of the electric guitar has powerful and characteristic sound quality and is persistently favored by performers. For example, in the case that the speaker is driven by a vacuum tube amplifier with a maximum output of 100 W, a peak voltage close to 90 V is applied to the speaker in the frequency band near the resonance frequency (f0), whereby powerful sound can be emitted at a large volume.
For example, in order that a speaker is driven by a class D amplifier and that a sufficient load current is supplied to the speaker in the frequency band near the resonance frequency (f0), it is necessary to use a high output class D amplifier having a high power source voltage at its output stage. However, in the case that the speaker is driven by such a high output class D amplifier, constant voltage drive is performed even in a frequency band in which the impedance of the speaker is low, whereby excessive current flows in the speaker and the speaker may be broken.
As described above, such a vacuum tube amplifier has excellent advantages as a guitar amplifier. However, the vacuum tube amplifier is large in size and heavy in weight, thereby being inconvenient in handling. Furthermore, a pentode having high output impedance is used as the output stage in the vacuum tube amplifier, the loss in the output stage is large, and there is a problem of being unable to efficiently drive a load (speaker).